Phosphor particles are used in a variety of applications such as flat panel displays and decorations, cathode ray tubes, and fluorescent lighting fixtures. Luminescence or light emission by phosphor particles may be stimulated by application of heat (thermoluminescence), light (photoluminescence), high energy radiation (e.g., x-rays or e-beams), or electric fields (electroluminescence).
Electroluminescent ("EL") phosphors are of particular commercial importance. The luminescent brightness and "maintenance" of such brightness of such phosphors are two important criteria for characterizing phosphor particles. Luminescent brightness is typically reported as a quantity of light emitted by the subject phosphor when excited. When reported in absolute brightness, e.g., in foot-Lamberts ("ft-L"), the conditions under which the phosphor is excited should also be reported as the absolute luminescent brightness of a given phosphor typically depends upon a combination of several factors. For instance, the absolute brightness of an electroluminescent phosphor should be reported with specified voltage and frequency of the applied electric field and temperature of the phosphor. The luminescent brightness attained is also dependent in part upon the physical characteristics and specifications of the test device used to measure the magnitude of emitted light. A typical test device possesses many of the same members as the thick film electroluminescent devices discussed below. With regard to accurately determining the absolute brightness of a subject phosphor, important characteristics thereof include the thickness of the phosphor layer, the concentration or loading of the phosphor particles in the dielectric matrix, the characteristics of the particular dielectric matrix material, and the transparency of the front electrode. Because of the sensitivity of phosphor emission brightness to varying conditions of excitement, the brightness of phosphors are more typically reported as relative brightnesses rather than as absolute brightness. "Maintenance" refers to the rate at which phosphors lose brightness during operation. As discussed, by Thornton in Electroluminescent Maintenance, Jour. of Electrochem. Soc., pp 895-907, Vol. 107, No. 11, November 1960, such a decrease in brightness with operating time is a typical characteristic of phosphors. Furthermore, the rate of decay is substantially increased if the phosphor particles are subjected to conditions of high humidity while being operated. "Atmospheric water vapor is perhaps the most important adverse influence on electroluminescence maintenance from the point of view of practical application." Ibid. This effect of moisture or high humidity is referred to herein as "humidity-accelerated decay".
Decay characteristics observed during operation at zero relative humidity are referred to as the intrinsic maintenance characteristics or intrinsic decay of the subject phosphor. The intrinsic decay varies with operating conditions such as voltage, frequency, and temperature, but is essentially reproducible for a given phosphor for a given set of operating conditions. As noted by Thornton, operation in high humidity, e.g., relative humidity of greater than about 80 percent can increase the decay rate by a factor of 10 or more with respect to the subject phosphor's intrinsic decay.
Particulate EL phosphors are most commonly used in thick film constructions. These devices typically include a layer of an organic dielectric matrix, e.g., polyester, polyethylene terephthalate, cellulosic materials, etc., preferably having a high dielectric constant, loaded with phosphor particles, e.g., sulfide-based phosphor particles. Such layers are typically coated on a plastic substrate having a transparent front electrode. A rear electrode, e.g., an aluminum foil or screen printed silver ink, is typically applied to the back side of the phosphor layer. When an electric field is applied across the electrodes, the proximate portions of the layer emit light as the phosphor particles therein are excited. Such constructions may further comprise optional dielectric layers between the phosphor layer and rear electrodes.
Organic matrices and coatings can temporarily delay or slow the rate of humidity-accelerated decay, however, after moisture permeates the matrix or coating, rapid loss of luminescent brightness is typically exhibited. Organic matrices and substrate materials have typically been insufficiently effective in preventing diffusion of water vapor to the phosphor particles, and have accordingly been ineffective in preventing subsequent decay of brightness. For this reason, thick film electroluminescent devices are typically encased in relatively thick, e.g., 25 to 125 microns, envelopes of moisture-resistant materials such as fluorochlorocarbon polymers such as ACLAR Polymers from Allied Chemical. Some of the problems with such envelopes include typically substantial expense, unlit borders, and potential for delamination, e.g., under heat.
U.S. Pat. No. 4,097,776 (Allinikov) discloses electroluminescent phosphors coated with a liquid crystal in a solution-based technique. U.S. Pat. No. 4,508,760 (Olson et al.) discloses encapsulation of electroluminescent phosphors via vacuum deposition of certain polymers.
It is also known to encapsulate phosphor particles in inorganic coatings, e.g., oxide coatings. U.S. Pat. No. 3,264,133 (Brooks) discloses the deposition of coatings such as titania (TiO.sub.2) on phosphor particles by washing the particles in a predominantly alcohol solution of a halogen-containing constituent, e.g., titanium tetrachloride, and then drying and firing the particles.
Vapor phase reaction and deposition processes have been used to coat phosphor particles with inorganic coatings. Such techniques are typically considered as superior in providing more complete, uniform, and defect-free coatings. Phosphor particles encapsulated with such techniques have exhibited substantial resistance to humidity-accelerated decay. However, significant reductions in humidity-accelerated decay of luminescent brightness have been obtained only in conjunction with greatly diminished initial luminescent brightness and in some instances, undesirable color shift of the light emitted by the encapsulated phosphor particles.
For instance, U.S. Pat. No. 4,855,189 (Simopoulous et al.) discloses encapsulation of phosphor particles with SiO.sub.2 via a chemical vapor deposition process ("CVD") wherein phosphor particles are subjected to a temperature of about 490.degree. C. and an atmosphere of oxygen and silane gas while being agitated. Phosphor particles encapsulated in accordance with this reference have been found to exhibit a substantial reduction in initial electroluminescent brightness for given excitement conditions.
Air Force Technical Report AFFDL-TR-68-103 (Thompson et al., July 1968) discloses vapor phase encapsulation of electroluminescent phosphor particles for the purpose of attempting to improve performance at elevated temperatures. That reference discloses use of a fluidized bed chemical vapor deposition ("CVD") process to deposit several different oxide coatings onto zinc sulfide-based phosphors. Oxide coatings were deposited from a variety of precursor materials at furnace settings of about 200.degree. C. to about 500.degree. C. The reactor temperature profile was such that the maximum temperature within the reaction zone was typically 100.degree. C. higher than the nominal temperature setting, accordingly, the maximum temperatures within the reactor ranged upward of about 300.degree. C. for the various deposition runs disclosed therein. Titania-coated zinc sulfide/zinc selenide phosphors were found to have a reduced humidity-accelerated decay, but the initial luminescent brightness of the encapsulated phosphors was only about 25 percent that of the original material in uncoated form.
The prior art does not disclose a technique for encapsulating phosphor particles that provides desired moisture-resistance coupled with high levels of initial luminescent brightness relative to the initial luminescent brightness of the uncoated phosphor particles.